Frequency hopping

ABSTRACT

A cellular communications system includes a base station and a plurality of user devices, wherein each user device is operable to communicate with the base station over a communication channel having a plurality of frequency resources, wherein the base station is operable: i) to provide each user device with a respective initial allocation of the frequency resources; and ii) to provide at least one user device with a periodic communications opportunity in which the user device can communicate with the base station, wherein each user device is operable to apply a frequency shift to its initially allocated frequency resource in accordance with a frequency hopping sequence, wherein the user devices use the same frequency hopping sequence and are synchronized with each other so that, at any point in time, a common frequency shift is applied by the user devices.

The present application is a Divisional Application of U.S. patentapplication Ser. No. 12/521,298, filed on Jun. 25, 2009, which is basedon British Patent Application No. 0702190.0, filed on Feb. 5, 2007, theentire contents of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to frequency hopping within acommunication system. The invention has particular, although notexclusive relevance to the efficient signalling of frequency hoppinginformation between devices of a communication system that usesFrequency Division Multiple Access (FDMA) techniques.

This application is based upon and claims the benefit of priority fromUnited Kingdom Patent application No. 0702190.0, filed on Feb. 5, 2007,the disclosure of which is incorporated herein in its entirety byreference.

BACKGROUND ART

Localised FDMA with inter and intra Transmission Time Interval (TTI)Frequency Hopping (L-FDMA+FH) has been selected as the uplink multipleaccess scheme for the E-UTRA air interface currently been studied in3GPP (which is a standard based collaboration looking at the futureevolution of third generation mobile telecommunication systems). Underthe E-UTRA system, a base station (eNodeB) which communicates with anumber of user devices (UEs) allocates the total amount oftime/frequency resource among as many simultaneous users as possible, inorder to enable efficient and fast link adaptation and to attain maximummulti-user diversity gain.

DISCLOSURE OF INVENTION

In such communication systems, problems arise in determining how toperform the frequency hopping in the different user devices and how tosignal a selected frequency hopping scheme to each user device so thatthey know which resources to use for their communications.

According to one aspect, the present invention provides a communicationsnode operable to communicate with another communications node over acommunications channel having a plurality of frequency resources, thecommunications node comprising: a resource determination module operableto apply a frequency shift to an initially allocated frequency resourcein accordance with a frequency hopping sequence to determine a frequencyresource to use for communicating data with the other communicationsnode, the frequency shift corresponding to an integer number offrequency sub-bands of the communications channel. The communicationnode in this case may be a user device such as a mobile telephone or alaptop computer etc or a base station of a cellular communicationsnetwork.

In one exemplary embodiment each frequency resource in a sub-band has acorresponding frequency resource in each of the other sub-bands and theresource determination module is operable to apply a frequency shiftthat moves the initially allocated frequency resource to a correspondingfrequency resource in another sub-band.

Preferably, if the initial allocation comprises a plurality of saidfrequency resources, these are contiguous in the same sub-band, so thatthe shifted resources are also in the same sub-band. This is preferredas it allows the communications node to be able to transmit informationmore efficiently than would be the case if the shifted resources are notcontiguous and lie in different sub-bands.

In one exemplary embodiment, the resource determination module applies afrequency shift to its initially allocated frequency resource inaccordance with a pseudo-random frequency hopping sequence, which may becalculated in advance or dynamically calculated at the time that theshift is to be applied using a predefined equation. Such an equationpreferably uses a pseudo-random value so that the frequency hoppingsequence obtained appears random.

In one exemplary embodiment, when the above communications node is abase station that communicates with a number of other communicationsnodes, it maintains data defining an initial allocation of saidfrequency resources for each of said other communications nodes; and theresource determination module applies a common frequency shift to theinitially allocated frequency resource for each other communicationsnode to determine a respective frequency resource to use forcommunicating information with each other communications node.

This aspect of the present invention also provides a communicationssystem comprising: a communications node and a plurality of user devicesoperable to communicate with the communications node over acommunications channel; wherein the communications channel includes atransmission bandwidth, having a plurality of frequency resources, thatis divided into a plurality of contiguous sub-bands each having Nfrequency resources; wherein each user device has a respective initialallocation of said frequency resources to use for communications withsaid communications node; and wherein each user device is operable toapply a frequency shift to its initially allocated frequency resource inaccordance with a frequency hopping sequence, the frequency shiftcorresponding to an integer number of said sub-bands.

According to another, different, aspect, a cellular communicationssystem is provided comprising: a base station and a plurality of userdevices; wherein each user device is operable to communicate with thebase station over a communication channel having a plurality offrequency resources; wherein the base station is operable: i) to provideeach user device with a respective initial allocation of said frequencyresources; and ii) to provide at least one user device with a periodiccommunications opportunity in which the user device can communicate withthe base station; wherein each user device is operable to apply afrequency shift to its initially allocated frequency resource inaccordance with a frequency hopping sequence; wherein the user devicesuse the same frequency hopping sequence and are synchronised with eachother so that, at any point in time, a common frequency shift is appliedby the user devices; and wherein the frequency hopping sequence used bythe user devices is periodic and has a period that is greater than theperiodicity of the periodic communications opportunity provided to saidat least one user device. In this way, some frequency diversity will beprovided for the at least one user device having the periodiccommunications opportunity.

In one exemplary embodiment, the at least one user is a persistentlyscheduled user and wherein one or more of the other user devices aredynamically scheduled users. Where several persistently scheduled userdevices are provided with different communications intervals, the periodof the frequency hopping sequence is set to be greater than the longestcommunications interval.

The frequency hopping sequence used by the user devices preferably has aperiod that is greater than five times and more preferably greater thanten times the periodicity of the periodic communications opportunityprovided to said at least one user device. Where the communicationschannel is divided into a plurality of contiguous sub-bands, thefrequency hopping sequence used by the user devices may have a periodthat is greater than the periodicity of the periodic communicationsopportunity provided to said at least one user device times the numberof said sub-bands.

In one exemplary embodiment each user device applies a frequency shiftto its initially allocated frequency resource in accordance with apseudo-random frequency hopping sequence, which may be fixed in advanceor dynamically calculated using a predetermined equation. Preferably theuser devices dynamically calculate the frequency shift to be applied ata given time point using an equation that involves a pseudo-randomvalue, as this makes the frequency hopping sequence appear random. Ashift register circuit may be used for generating, at each time point,said pseudo-random value. In one exemplary embodiment, the shiftregister circuit has M registers and can generate a sequence of pseudorandom values up to 2^(M) in length and wherein the user devicesperiodically reset the shift register in synchronism with theperiodicity of the frequency hopping sequence. When resetting the shiftregister, the user device preferably controls the frequency hoppingsequence to be used by setting an initial state of the shift registereach time it is reset to one of a predetermined number of possibleinitialisation states. This allows the same shift register circuit to beable to generate a number of different hopping sequences. In such anexemplary embodiment, the base station may signal which initialisationstate is to be used by each user device at any given time.

According to a still further aspect, a cellular communications system isprovided comprising: a plurality of base stations and a plurality ofuser devices; wherein, in use, each user device is associated with abase station and is operable to communicate with the associated basestation over a communication channel having a plurality of frequencyresources; wherein each user device has a respective initial allocationof said frequency resources; wherein each user device is operable toapply a frequency shift to its initially allocated frequency resource inaccordance with a frequency hopping sequence; wherein the user devicesthat are associated with the same base station are operable, in use, touse the same frequency hopping sequence and are synchronised with eachother so that, at any point in time, a common frequency shift is appliedby the user devices associated with the same base station; and whereinuser devices associated, in use, with different base stations usedifferent frequency hopping sequences. In this way, each base stationcan control the initial resource allocation provided to the user devicesassociated therewith to minimise transmission collisions between userdevices associated with the same base station and by using a differentfrequency hopping sequence in the user devices associated with differentbase stations, inter cell collisions can also be reduced.

As those skilled in the art will appreciate, the invention relates to anumber of different components of a system that can be made and soldseparately. The invention also extends to these components alone as wellas to the system as a whole.

As those skilled in the art will appreciate, the above aspects can beimplemented separately or in any combination in a communications system.A specific exemplary embodiment will be described below which appliesall the above aspects in a communications system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and various other aspects of the invention will become apparent,from the following detailed description of an exemplary embodiment whichis given by way of example only and which is described with reference tothe accompanying Figures in which:

FIG. 1 schematically illustrates a communication system comprising anumber of user mobile (cellular) telephones which communicate with oneof two base stations connected to the telephone network;

FIG. 2 illustrates the structure of a sub-frame of the E-UTRAcommunication system;

FIG. 3 is a block diagram illustrating a shift register used forgenerating a pseudo random binary sequence for controlling the frequencyhopping to be used by each user mobile telephone;

FIG. 4 is a time and frequency plot illustrating the way in which someof the available time and frequency resource blocks have been assignedto four mobile telephones;

FIG. 5 is a block diagram illustrating the main components of one of thebase stations shown in FIG. 1;

FIG. 6 is a block diagram illustrating the main components of one of themobile telephones shown in FIG. 1;

FIG. 7 is a block diagram illustrating a shift register arrangement likein FIG. 3; and

FIG. 8 is a time and frequency plot showing a hopping pattern for fourUEs (user devices) in a cell like in FIG. 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Overview

FIG. 1 schematically illustrates a mobile (cellular) telecommunicationsystem 1 in which users of mobile telephones 3-0, 3-1, and 3-2 in afirst cell 4-1 can communicate with other users (not shown) via a firstbase station 5-1 and a telephone network 7 and in which users of mobiletelephones 3-3, 3-4, and 3-5 in a second cell 4-2 can communicate withother users (not shown) via a second base station 5-2 and the telephonenetwork 7. In this exemplary embodiment, the base stations 5 use anorthogonal frequency division multiple access (OFDMA) transmissiontechnique for the downlink (from base stations 5 to the mobiletelephones 3) and a L-DMA+FH transmission technique for the uplink (fromthe mobile telephones 3 to the base stations 5).

The use of frequency hopping for the uplink has been chosen because itprovides service quality improvements through interference averaging andfrequency diversity. In this exemplary embodiment, the frequency hoppingscheme is chosen so that the following requirements are preferably met:

-   -   No collision between hopping mobile telephones 3 in the same        cell 4;    -   Different hopping patterns in neighbouring cells 4 to reduce        inter-cell interference;    -   High degree of frequency diversity for one mobile telephone 3        throughout the hopping pattern for subsequent transmissions;    -   Preserve the single carrier property of L-FDMA (in which the        allocated frequency resources are provided as a single        contiguous block of frequency resources);    -   Minimise the signalling overhead for informing the mobile        telephones 3 of the hopping sequence; and    -   Frequency hopping designed for use by persistently scheduled        mobile telephones 3 that are using, for example, services such        as VoIP, as well as mobile telephones 3 that are dynamically        scheduled on a TTI by TTI basis.

Time/Frequency Resources

In this exemplary embodiment, the available transmission band is dividedinto a number of sub-bands, each of which comprises a number ofcontiguous sub-carriers arranged in contiguous blocks. Different mobiletelephones 3 are allocated different resources block(s) (sub-carriers)within a sub-band at different times for transmitting their data. FIG. 2illustrates E-UTRA's latest definition of the transmission channel ascomprising a sequence of 1 ms Transmission Time Intervals (TTIs) 11-1,11-2, each of which consists of two 0.5 ms slots 13-1 and 13-2. Asshown, the available transmission bandwidth is divided into S sub-bands15-1 to 15-s and each mobile telephone 3 is scheduled to transmit itsuplink data in selected slots 13 and in selected sub-bands 15, inaccordance with the agreed frequency hopping sequence.

Two different types of frequency hopping can be applied—Inter TTIfrequency hopping and Intra TTI frequency hopping. Inter TTI frequencyhopping is when the allocated frequency resource is changed from one TTI11 to the next and intra TTI frequency hopping is where the allocatedfrequency resource is changed from one slot 13 to the next. Thetechnique to be described below is applicable to both Inter and IntraTTI frequency hopping, although the description will refer mainly toInter TTI frequency hopping.

Proposed Frequency Hopping Scheme

The frequency hopping scheme used in this exemplary embodiment relies oneach mobile telephone 3 being given an initial allocation of resourceblocks (one or more contiguous blocks of sub-carriers) within one of thesub-bands. These initial allocations are assigned by the base station 5,and so it can make sure that there are no collisions between the initialallocations for the mobile telephones 3 within its cell 4. These initialallocations are then changed in accordance with a hopping sequenceallocated to the cell 4. The change applied at any point in time is aninteger multiple of the number of resources in each sub-band. As aresult, the frequency hopped resources that are allocated to a mobiletelephone 3 will also be a contiguous block of resources in a singlesub-band. This is beneficial as it allows the power amplifier (notshown) used by the mobile telephones 3 to be more efficient than wouldbe the case if the resources used are not contiguous and are not in thesame sub-band. It follows that, to maintain this advantage, the largestallowable contiguous allocation for a hopping mobile telephone 3corresponds to the number of resource blocks in a sub-band.

Mathematically, the frequency hopping scheme used in this exemplaryembodiment can be defined as follows:y={x+a(t)N} mod N _(RB)  Equation 1,where

N_(RB) is the total number of resource blocks in the transmission band;

N is the number of contiguous resource blocks in each sub-band;

x is the initial resource block allocated to the mobile telephone;

y is the frequency hopped resource block;

t is a TTI (or slot) counter synchronised between the base station 5 andthe mobile telephone 3;

a(t) is the current frequency hopping shift and is an integer value fromthe set {0, 1, . . . , S−1}; and

S is the number of sub-hands.

FIG. 3 illustrates a shift register used for generating a pseudo randombinary sequence for controlling the frequency hopping to be used by eachuser mobile telephone. The shift register of FIG. 3 will later bedescribed.

FIG. 4 illustrates a hopping pattern that can be generated in the abovemanner for four mobile telephones (MTs), where MT1 to MT3 are assignedone resource block each while MT4 is assigned two resource blocks. Inthis example, a(t) has values of 0, 2, S−1 and 1 for TTI#0, TTI#1, TTI#2and TTI#n respectively.

The way in which contiguous resource blocks can be allocated for theuplink and signalled to User Equipment (such as the mobile telephones 3)has already been proposed in TSG-RAN R1-070364, “Uplink ResourceAllocation for EUTRA” NEC Group, NTT DoCoMo, the contents of which areincorporated herein by reference. As those skilled in the art willappreciate, if a mobile telephone 3 is assigned more than one resourceblock (x), then the calculation above is performed for each assignedresource block.

In this exemplary embodiment, N_(RB), N and S are system semi-staticconstants and are programmed into the mobile telephones 3 and the basestations 5 in advance. At any given time, the allocated resource block,x, is different for each of the mobile telephones 3 in the same cell 4.However, the value of a(t) at any point in time is common for all mobiletelephones 3 in the same cell 4 and the value is changed in accordancewith a predetermined hopping sequence. The hopping sequence preferablyhas the following properties:

-   -   1. It should be different in different cells 4 in order to        randomise inter-cell interference;    -   2. It should be simple to generate (to minimise computational        load in the base stations 5 and the mobile telephones 3);    -   3. It should be defined by a small number of parameters (to        minimise signalling load); and    -   4. It should be periodic with a period, T, that is much longer        than the transmission interval of persistently scheduled users        (otherwise there is a risk that the transmission interval is        equal to the period of a(t), in which case there would be no        frequency diversity).

In the event that some TTIs are set aside for hopping mobile telephones3, the hopping shift a(t) would only be applied in those TTIs.Dynamically scheduled mobile telephones 3 may still be scheduled in such‘hopping TTIs’ in any resource blocks which are not occupied by thehopping mobile telephones 3.

There are a number of different ways of generating a(t) in the mobiletelephones 3 and the base station 5. One possibility is use apseudo-random sequence, resetting the sequence every T TTIs (or slots).A large number of sequences could easily be generated in this way andthe sequence number could be signalled efficiently. For example,consider the shift register arrangement 17 shown in FIG. 3, whichproduces a length 2047 pseudo-random binary sequence (PRBS). The stateof the shift register 17 is updated each TTI (or slot). If the 11 bitshift register value at time t is represented by m(t), then apseudo-random value in the range 0 to S−1 can be calculated, forexample, as follows:a(t)=floor[m(t)·S)/2048]  Equation 2,where floor[r] is the floor function, ie the largest integer not greaterthan r.

This calculation is easy to perform using a multiplication and bitshift. By resetting the shift register every T=256 TTIs (or slots),eight different sequences can be produced using different initialstates. More specifically, the shift register 17 shown in FIG. 3 cyclesthrough 2047 states that we can label s(0) to s(2046). As the registersare being reset every 256 TTIs (or slots), the register will only cyclethrough 256 of its 2047 possible states. Therefore, it is possible touse the same shift register 17 to generate different a(t) sequences,simply by starting the shift register 17 at different initial states.For example, a first a(t) sequence can be defined by setting the shiftregister 17 into initial state s(0); a second a(t) sequence can bedefined by setting the shift registers 17 into initial state s(256); athird a(t) sequence can be defined by setting the shift registers 17into initial state s(512) etc. Different sequences can then be assignedto the base station 5 and the mobile telephones 3 in the different cells4, thereby avoiding the possibility that two mobile telephones 3 indifferent cells 4 could be following exactly the same frequency hoppingpattern and therefore colliding 100% of the time. The mobile telephones3 in a given cell 4 may be signalled with the initial state, but thiswould require eleven bits of signalling overhead. Therefore, in thisexemplary embodiment, all the initial states are pre-programmed into themobile telephones 3 and the appropriate one to be used by the mobiletelephones 3 in a cell are signalled to the mobile telephones 3 using anassociated sequence identifier (which would be a 3-bit identifier forthe above example having eight different sequences).

Base Station

FIG. 5 is a block diagram illustrating the main components of each ofthe base stations 5 used in this exemplary embodiment. As shown, eachbase station 5 includes a transceiver circuit 21 which is operable totransmit signals to and to receive signals from the mobile telephones 3via one or more antennae 23 and which is operable to transmit signals toand to receive signals from the telephone network 7 via a networkinterface 25. The operation of the transceiver circuit 21 is controlledby a controller 27 in accordance with software stored in memory 29. Inthis exemplary embodiment, the software in memory 29 includes, amongother things, an operating system 31, a resource allocation module 33and a resource determination module 34 (which modules may form part ofthe operating system 31).

The resource allocation module 33 is operable for allocating initialresource blocks (x) to be used by each of the mobile telephones 3 intheir communications with the base station 5. This initial resourceallocation depends on the type and quantity of data to be transmitted bythe user device. For users subscribing to services where regular butsmall amounts of data are to be transmitted, the resource allocationmodule 33 allocates appropriate resource blocks on a recurring orperiodic basis. For a VoIP service, for example, this may result in theuser being allocated resource blocks every 20 ms. This type ofallocation is referred to as persistent allocation. For users withlarger volumes of data to be transmitted, the resource allocation modulewill allocate the appropriate resource blocks on dynamic basis, takinginto account the current channel conditions between the user's mobiletelephone 3 and the base station 5. This type of allocation is referredto as dynamic allocation.

The resource determination module 34 is provided for determining theactual frequency resources that each mobile telephone 3 will use totransmit its data to the base station 5. The resource determinationmodule 34 uses the determined frequency resources to control theoperation of the transceiver circuit 21 so that the data received fromeach mobile telephone 3 can be recovered and forwarded as appropriate tothe telephone network 7. To achieve this, the resource determinationmodule 34 implements the above described shift register 17-5 and TTI (orslot) counter (t) 35 (although these could be implemented as hardware inthe controller 27), so that it can keep track of which resource block orblocks will actually be used by each mobile telephone 3 at each point intime, using equations 1 and 2 above and the initial allocations made bythe resource allocation module 33. In this exemplary embodiment, theresource determination module 34 receives a sequence identifier from thetelephone network 7 identifying the initial state to be applied to itsshift register 17-5. The resource determination module 34 uses thesequence identifier to retrieve the corresponding initial state frommemory which it then uses to set the initial state of the shift register17-5. The resource determination module 34 also signals the receivedsequence identifier to all the mobile telephones 3 in its cell 4. Theresource determination module 34 also transmits synchronisation data tosynchronise the TTI (or slot) counters in the mobile telephones 3 withits own TTI (or slot) counter 35, so that the base station 5 and themobile telephones 3 can maintain synchronisation in applying thefrequency hopping sequence (a(t)).

Mobile Telephone

FIG. 6 schematically illustrates the main components of each of themobile telephones 3 shown in FIG. 1. As shown, the mobile telephones 3include a transceiver circuit 71 which is operable to transmit signalsto and to receive signals from the base station 5 via one or moreantennae 73. As shown, the mobile telephone 3 also includes a controller75 which controls the operation of the mobile telephone 3 and which isconnected to the transceiver circuit 71 and to a loudspeaker 77, amicrophone 79, a display 81, and a keypad 83. The controller 75 operatesin accordance with software instructions stored within memory 85. Asshown, these software instructions include, among other things, anoperating system 87 and a resource determination module 89. In thisexemplary embodiment, the resource determination module 89 includes theabove described 11-bit shift register 17-3 and a TTI (or slot) counter91.

In operation, the resource determination module 89 receives the sequenceidentifier for the cell 4 transmitted by the base station 5 in a commonsignalling channel. The resource determination module 89 uses thissequence identifier to retrieve the corresponding initial state to beapplied to the shift register 17-3 from memory. The resourcedetermination module 89 also receives the synchronisation data forsynchronising its TTI (or slot) counter 91 with the correspondingcounter 35 in the base station 5. In this exemplary embodiment, themobile telephone 3 receives this information at the time that it firstassociates with the base station 5. The resource determination module 89also receives resource allocation data identifying the initiallyallocated resources, x, as well as the TTI 11 and/or the slot 13 inwhich those resources have been allocated to that mobile telephone 3.For persistently scheduled mobile telephones 3, this resource allocationdata may define a period between allocated TTIs or slots, such that themobile telephone 3 is allocated resource block x every Y TTIs (orslots). In this case, the resource allocation data only has to betransmitted once or whenever the allocation is to be changed. Fordynamically scheduled users, the resource allocation data must betransmitted before each scheduled transmission.

Once the resource determination module 89 has received the data toinitialise the shift register 17-3 and the counter 91 as well as theresource allocation data, it uses equations 1 and 2 to determine theactual resource block(s) to use for its uplink transmissions in thescheduled TTI (or slot). This information is then used to control theoperation of the transceiver circuit 71 accordingly.

Modifications and Alternatives

A detailed exemplary embodiment has been described above. As thoseskilled in the art will appreciate, a number of modifications andalternatives can be made to the above exemplary embodiment whilst stillbenefiting from the inventions embodied therein. By way of illustrationonly a number of these alternatives and modifications will now bedescribed.

In the above exemplary embodiment, equation 2 was used to generate thevalue of a(t) to be used in equation 1. If required, this calculationcould be modified slightly to ensure that successive values of a(t) arealways different, as follows:a(t)={a(t−1)+1+floor[(m(t)·(S−1))/2^(M)]} mod S  Equation 3,where a(0)=0 and M is the number of registers in the shift register 17.

Another possibility is to generate a(t) by cyclic sampling of thesequence 0, 1, . . . , S−1 as follows:

a(t)=kt mod S t=0 to T−1,

where k is an integer co-prime to S. In this case, different values of kyield different sequences. However, since the resulting sequence will beperiodic with period S, it is unlikely to meet the desired requirementthat its period is much longer than the transmission interval ofpersistently scheduled users.

In the above exemplary embodiment, the base station 5 received thesequence identifier from the telephone network 7 which identified theinitialisation state to be applied to its shift register 17-5. Thisallocation of the initialisation states may be fixed for the network orit may be changed on a regular or periodic basis. If it is changed, thebase station 5 preferably broadcasts the new initialisation state (orstate identifier) in a common signalling channel so that the mobiletelephones 3 can update their shift registers 17-3 accordingly. In oneexemplary embodiment, the base stations 5 may be arranged to randomlyselect an initialisation state to use. In this case it is possible thattwo neighbouring cells 4 could end up using the same hopping sequence,but by changing the sequence regularly or periodically it is possible toensure that any resulting inter-cell interference will be short lived.

In the above exemplary embodiment, 11-bit shift registers were used ingenerating the appropriate frequency hopping sequence. As those skilledin the art will appreciate, longer or shorter length shift registerscould be used instead. Similarly, the number of different sequences thatcan be obtained from the shift register can also be varied—it does nothave to be eight. As those skilled in the art will appreciate, for agiven length of shift register, there is a trade off between the numberof sequences that can be derived from it and the periodicity (T) ofthose sequences. The length of the sequence is preferably at least 5times and more preferably more than 10 times longer than thetransmission interval of any persistently scheduled users. To ensuremaximum frequency diversity for all users, the length of the sequenceshould correspond to the length of the maximum transmission intervalmultiplied by the number of sub-bands (S).

In the above exemplary embodiment, a mobile telephone basedtelecommunication system was described in which the above describedfrequency hopping techniques were employed. As those skilled in the artwill appreciate, many of these frequency hopping techniques can beemployed in any communication system that uses a plurality of resourceblocks. In particular, many of these frequency hopping techniques can beused in wire or wireless based communication systems which either useelectromagnetic signals or acoustic signals to carry the data. In thegeneral case, the base stations and the mobile telephones can beconsidered as communications nodes which communicate with each other.The frequency hopping techniques described above may be used just foruplink data, just for downlink data or for both downlink and uplinkdata. Other communications nodes may include user devices such as, forexample, personal digital assistants, laptop computers, web browsers,etc.

In the above exemplary embodiments, a number of software modules weredescribed. As those skilled will appreciate, the software modules may beprovided in compiled or un-compiled form and may be supplied to the basestation or to the mobile telephone as a signal over a computer network,or on a recording medium. Further, the functionality performed by partor all of this software may be performed using one or more dedicatedhardware circuits. However, the use of software modules is preferred asit facilitates the updating of base station 5 and the mobile telephones3 in order to update their functionalities.

In the above exemplary embodiments, certain system constants such as thetotal number of resource blocks in the communication channel, the numberof sub-bands and the number of resource blocks in each sub-band wereprogrammed into the mobile telephones and the base stations. Thisinformation may be programmed directly into the software instructionsrun on these devices or may be software inputs that can be varied fromtime to time. In either case, the mobile telephones and the base stationwill include data (software or inputs) that define these systemconstants either directly or indirectly. For example, data may be storedthat directly defines the values of N_(RB) and S together with datadefining how N can be derived from these two.

The following is a detailed description of the way in which the presentinventions may be implemented in the currently proposed 3GPP LTEstandard. Whilst various features are described as being essential ornecessary, this may only be the case for the proposed 3GPP LTE standard,for example due to other requirements imposed by the standard. Thesestatements should not, therefore, be construed as limiting the presentinvention in any way. The following description will use thenomenclature used in the Long Term Evolution (LTE) of UTRAN. Forexample, a base station is referred to as eNodeB and a user device isreferred to as a UE.

1 INTRODUCTION

During TSG-RAN WG1 #46bis discussions, it was decided that LocalisedFDMA (L-FDMA) with inter and intra TTI frequency hopping (L-FDMA+FH)would be used for EUTRA Uplink. However, there was not any discussionabout what kind of frequency hopping pattern can be supported by EUTRAUplink.

In this contribution, we collect some requirements that can be used forthe selection of an efficient hopping pattern for L-FDMA uplink andpropose a suitable frequency hopping scheme for the uplink.

2 REQUIREMENTS FOR FREQUENCY HOPPING PATTERN

It is well-known that frequency hopping provides service qualityimprovement through interference averaging and frequency diversity.However, frequency hopping needs to be tailored for each system. Thefollowing requirements are applicable to the LTE system [5-6]:

-   -   No collision between hopping UEs in the same cell;    -   Different hopping patterns in neighbouring cells to reduce        inter-cell interference;    -   High degree of frequency diversity for one UE throughout hopping        pattern for the subsequent transmissions;    -   Preserve the single carrier property of the L-FDMA;    -   Signalling overhead for informing UEs of a specific or common        hopping sequence should be kept as small as possible;    -   Frequency hopping should be designed for small sized packets        intended to persistent scheduled UEs (e.g. VoIP service) as well        as high speed UEs.

3 FREQUENCY HOPPING SCHEME

Let N_(RB) be the total number of Resource Blocks (RBs) in the wholebandwidth. Let the bandwidth be divided into S sub-bands of N=N_(RB)/Scontiguous RBs each.

If a UE is assigned a RB x, it is understood that the RB actually usedfor transmission in TTI (or slot) number t isy=x+a(t)N mod N _(RB),where

t is a TTI (or slot) counter synchronised between the eNodeB and UE; and

a(t) is a value from the set {0, 1, . . . , S−1}.

If a UE is assigned more than one RB, then the calculation above isperformed for each assigned RB. Provided that all the assigned RBs arecontiguous and contained within one of the S sub-bands, the singlecarrier property is retained even after applying the frequency hoppingshift a(f). It follows that the largest allowable contiguous allocationfor a hopping UE is N RBs. The signalling of the assigned contiguousresource allocations have already been proposed in [7].The periodic sequence a(t) is common for all UEs in the cell, and shouldhave the following properties.

-   -   5. It should be different in different cells in order to        randomise inter-cell interference.    -   6. It should be simple to generate (to minimise computational        load in the eNodeB and UE).    -   7. It should be defined by a small number of parameters (to        minimise signalling load).    -   8. Its period, T, should be much longer than the transmission        interval of persistently scheduled users (otherwise there is a        risk that the transmission interval is equal to the period of        a(t), in which case there would be no frequency diversity).

In the case that some TTIs are set aside for hopping UEs, the hoppingshift a(t) would only be applied in those TTIs. Dynamically scheduledUEs may still be scheduled in such ‘hopping TTIs’ in any RBs which arenot occupied by hopping UEs.

One possibility is to generate a(t) using a pseudo-random sequence,resetting the sequence every T TTIs (or slots). A large number ofsequences could easily be generated in this way and the sequence numbercould be signalled efficiently. For example, consider the shift registerarrangement which is shown in FIG. 7 and which produces a length 2047pseudo-random binary (PRBS) sequence.

The shift register state is updated each TTI (or slot). Let m(t)represent the 11-bit shift register value at time t. A pseudo-randomvalue in the range 0 to S−1 can be obtained as follows:a(t)=floor[(m(t)·S)/2048].

This calculation is easy to perform using a multiplication and bitshift. By resetting the shift register every T=256 TTIs (or slots), 8different sequences can be produced using different initial states.Obviously a longer shift register could produce more sequences, and/or alarger period T. These different sequences can also be assigned intodifferent cells.

If required, the calculation above could be modified slightly to ensurethat successive values of a(t) are always different, as follows:a(t)={a(t−1)+1+floor[(m(t)·(S−1))/2048]} mod S,where a(0)=0.

FIG. 8 shows a hopping pattern for four UEs where UE1 to UE3 areassigned 1RB each while UE4 is assigned 2RBs. In this example, a(t) hasvalues of 0, 2, S−1 and 1 for TTI#0, TTI#1, TTI#2 and TTI#nrespectively.

4 CONCLUSIONS

This contribution outlines some requirements for the selection of anefficient hopping pattern for L-FDMA uplink. In addition, a method forgenerating hopping patterns has been described for L-FDMA which avoidscollision between hopping UEs and at the same time mitigates other cellinterference.

Hence, we propose such frequency hopping scheme to be adopted for E-UTRAUplink.

5 REFERENCES

-   [1] TSG-RAN WG1#47, R1-063319 “Persistent Scheduling in E-UTRA”, NTT    DoCoMo, NEC Group.-   [2] TSG-RAN WG1 LTE AdHoc, R1-060099, “Persistent Scheduling for    E-UTRA” Ericsson.-   [3] TSG-RAN WG1#47, R1-063275, “Discussion on control signalling for    persistent scheduling of VoIP”, Samsung.-   [4] TSG-RAN WG1#44, R1-060604 “Performance Comparison of Distributed    FDMA and Localised FDMA with Frequency Hopping for EUTRA Uplink”,    NEC Group.-   [5] TSG-RAN WG1#46Bis, R1-062761 “Performance of D-FDMA and L-FDMA    with Frequency Hopping for EUTRA Uplink”, NEC Group, NTT DoCoMo.-   [6] TSG-RAN WG1#46Bis, R1-062851 “Frequency hopping for E-UTRA    uplink”, Ericsson.-   [7] R1-070364, “Uplink Resource Allocation for EUTRA” NEC Group, NTT    DoCoMo.

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, the invention is not limitedto these embodiments. It will be understood by those of ordinary skillin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asdefined by the claims.

This application is based upon and claims the benefit of priority fromUnited Kingdom patent application No. 0702190.0, filed on Feb. 5, 2007,the disclosure of which is incorporated herein in its entirety byreference.

1. A cellular communications system comprising: a base station and aplurality of user devices, wherein each user device is operable tocommunicate with the base station over a communication channel having aplurality of frequency resources, wherein the base station is operable:i) to provide each user device with a respective initial allocation ofsaid frequency resources; and ii) to provide at least one user devicewith a periodic communications opportunity in which the user device cancommunicate with the base station, wherein each user device is operableto apply a frequency shift to its initially allocated frequency resourcein accordance with a frequency hopping sequence, wherein the userdevices use the same frequency hopping sequence and are synchronizedwith each other so that, at any point in time, a common frequency shiftis applied by the user devices, and wherein the frequency hoppingsequence used by the user devices is periodic and has a period that isgreater than a periodicity of the periodic communications opportunityprovided to said at least one user device.
 2. A cellular communicationssystem according to claim 1, wherein the frequency hopping sequence usedby the user devices has a period that is greater than five times theperiodicity of the periodic communications opportunity provided to saidat least one user device.
 3. A cellular communications system accordingto claim 1, wherein the frequency hopping sequence used by the userdevices has a period that is greater than ten times the periodicity ofthe periodic communications opportunity provided to said at least oneuser device.
 4. A cellular communications system according to claim 1,wherein the communications channel is divided into a plurality ofcontiguous sub-bands and wherein the frequency hopping sequence used bythe user devices has a period that is greater than the periodicity ofthe periodic communications opportunity provided to said at least oneuser device times the number of said sub-hands.
 5. A cellularcommunications system according to claim 4, wherein each frequencyresource in a sub-band has a corresponding frequency resource in each ofthe other sub-bands and wherein each user device is operable to apply afrequency shift that moves the initially allocated frequency resource toa corresponding frequency resource in another sub-band.
 6. A cellularcommunications system according to claim 1, wherein the communicationschannel is divided into a plurality of contiguous sub-bands and whereinsaid user devices are operable to apply a frequency shift to itsinitially allocated frequency resource in accordance with the followingequation:y={x+a(t)N} mod N _(RB), where N_(RB) is the total number of frequencyresources in the transmission band; N is the number of contiguousfrequency resources in each sub-band; x is the initially allocatedfrequency resource; y is the frequency hopped resource; t is a timecounter; a(t) is the frequency hopping shift applied at time point t,and is an integer value from the set {0, 1, . . . , S−1}; and S is thenumber of sub-bands.
 7. A cellular communications system according toclaim 1, wherein said user devices are operable to apply a frequencyshift to its initially allocated frequency resource in accordance with apseudo-random frequency hopping sequence.
 8. A cellular communicationssystem according to claim 1, wherein said user devices are operable todynamically calculate the frequency shift to be applied at a given timepoint using a predetermined equation.
 9. A cellular communicationssystem according to claim 8, wherein the user devices are operable todynamically calculate the frequency shift to be applied at a given timepoint using an equation that involves a pseudo-random value.
 10. Acellular communications system according to claim 9, further comprisinga shift register circuit for generating, at each time point, saidpseudo-random value.
 11. A cellular communications system according toclaim 10, wherein the shift register circuit has M registers and cangenerate a sequence of pseudo random values up to 2^(M) in length andwherein said user devices are operable to periodically reset said shiftregister in synchronism with the periodicity of the frequency hoppingsequence.
 12. A cellular communications system according to claim 10,wherein the user device is operable to control the frequency hoppingsequence to be used by setting an initial state of said shift registereach time it is reset to one of a predetermined number of possibleinitialization states.
 13. A cellular communications system according toclaim 12, wherein said base station is operable to signal whichinitialization state is to be used by each user device.
 14. Acommunications method characterized by using a system according toclaim
 1. 15. A computer implementable instructions product comprisingcomputer implementable instructions for causing a programmable computerdevice to become configured as a user device adapted for use as one ofthe user devices of the cellular communications system of claim 1; or tobecome configured as a base station adapted for use as the base stationof the cellular communications system of claim 1.